As a technique to improve the image quality of a color image taken by a color imaging device, a technique to make the color of a specific object (such as flesh color, green of plants, and a blue sky) in a color image closer to the recorded color of that object and thereby to reproduce a desirable color has been proposed.
For example, Patent literature 1 discloses a technique relating to a color correction of a color image. In Patent literature 1, a representative color is extracted from an object area in an image and RGB correction parameters are determined by comparing that representative color with a predefined center color for correction. Then, each pixel is corrected by controlling the strength at which these correction parameters are applied according to the distance from the center color. Specifically, a technique to intensively correct the color of an object by converting the color information, i.e., the RGB values of each pixel in the color image into a hue, a saturation, and a lightness, calculating a distance between that color and the center color for correction in the color space, and adjusting the correction strength according to that distance has been proposed.
In this technique, a color correction is performed based on the addition/subtraction of correction parameters in the RGB color space. For example, in the case of flesh color of a human face, the RGB correction amount is calculated for each pixel according to the distance from the center color for correction. If the face area is to be lightened in whole, a correction parameter is added to or subtracted from the RGB values of each pixel located in substantially the entire face area according to the above-mentioned distance from the center color for correction. Further, Patent literature 2 discloses a technique relating to the detection of a face area in an input image.
Patent literature 3 discloses a color correction device and method in which when a color correction is made to spectral-color image data, the spectral color is converted into a color space having dimensions lower than the original dimensions; a color correction is made in the lower-dimensional color space; and a spectral color in appropriate dimensions is generated from the lower dimensions.
Further, Patent literature 4 discloses a technique to convert an original color space into a target color space between color systems having different reference white colors, while maintaining the way in which colors are viewed unchanged from that in the original color space. Specifically, the spectral power distribution characteristic of the original reference white is restored from the color temperature of the original reference white, i.e., the reference white of the original color space. Further, the spectral power distribution characteristic of the target reference white is restored from the color temperature of the target reference white, i.e., the reference white of the target color space. Then, the surface reflectance of a given color in the original color space is restored by using the tristimulus values of the given color, the spectral power distribution characteristic of the original reference white, and the color matching functions of human beings. Further, tristimulus values, which are a color in the target color space, are obtained based on the restored surface reflectance, the restored spectral power distribution characteristic of the target reference white, and the color matching functions of human beings.
Patent literature 5 discloses a technique to automatically make an excellent white correction for an important object in a natural image that is taken under various lighting environments. Specifically, the body surface color of a specific object is extracted, and optimal color correction parameters are set for the extracted body surface color. In this way, it is possible to automatically make a color correction for an important object in a natural image taken under various lighting environments. Note that the inventions disclosed in Patent literatures 3, 4 and 5 have completely different configurations from that of the present invention described below.
Patent literature 6 proposes to apply a modeling technique for the skin reflection of a human being to the rendering of a face image. In this method, a 3D (three-dimensional) shape of a face is obtained by scanning the face by a 3D scanner. Then, a plurality of face images are obtained by illuminating the face from different viewpoints in different directions. A total reflectance and a normal vector map are estimated by using the surface scan data and the image data. Then, a transmittancy map is obtained by scanning the under-surface reflectance using a fiber-optic spectrometer. The total reflectance is separated into two components of an under-surface scattering and a (specular) surface reflectance.